The objective of this grant is the construction and evaluation of a Positron Emission Tomograph (PET) with a large axial field of view of 20- 25 cm. This device will be ideally suited for whole body surveys necessary in the detection and staging of metastatic tumors; it will have twice the field of view and four times the sensitivity of current state- of-the-art PET scanners. This performance improvement possible it will be necessary to implement and evaluate a fully three-dimensional reconstruction algorithm. Preliminary results indicate that this algorithm will not only improve the image quality but it will also be possible to implement it given existing technology with clinically acceptable reconstruction times.